In the current model of muscular contraction, the thick and thin filaments slide across each other, giving rise to the various states of the muscle. On the molecular level, the events which provide the driving force for the sliding motion are largely unknown. Identification of these events and their mechanisms is important not only for a basic understanding of the contractile apparatus but also for information which may bear on the underlying causes of muscle diseases. We propose to investigate, using both contractile proteins and model fibers, the origin of mechanical thrust in contraction and to relate structural, biochemical, and hydrodynamic properties of the proteins to the contraction mechanism and to the physiological behavior of muscle. Molecular probes will be used for these investigations, with emphasis on both time-averaged fluorescence spectroscopy and nanosecond polarization spectroscopy. In addition, other non-contractile systems will be examined for general information bearing on the general problem of macromolecular conformation.